The Franssisi Four Chronicles

Synopsis

From Book 1:

A young human finds himself in the hands of Diles, as they call the aliens that conquered their world. Fortunately, his new Master is not one of the sadistic ones that he has heard about. And the young woman Zell is supposed to train as a bed pet turns out to be smart, brave and inventive.

However, his Master intends to gift Zell to a Dile who turns out to be the worst possible owner – and who becomes a formidable enemy.